Recently, the relationship between diseases and the body fat distribution has been studied. According to the study, even in a non-obese person as well as an obese person, visceral fat accumulation in the abdominal cavity (particularly, in the mesentery and/or in the greater omentum) positively correlates with values of serum cholesterol, triglyceride, and blood glucose measured by the glucose tolerance test. Further, the visceral fat accumulation also positively correlates with the systolic and diastolic blood pressures, and accordingly it highly relates to diseases such as hypertension, diabetes, and hyperlipemia [Fujioka, S., et al. Metabolism, 36 54-59, 1987; Matsuzawa, Y., et al. Progress in Obesity Research, 309-312, 1990]. These diseases are, therefore, thought to be cured or prevented by decreasing visceral fat, by inhibiting visceral fat accumulation, or improving body fat distribution [Bray, G. A., Obesity Research, 3, Suppl. 4, 425S-434S, 1995].
For decreasing body fat, treatments such as ergo-therapy, diet, behavior therapy, and pharmacotherapy are practically employed. In the pharmacotherapy, an anorectic agent working on the central nervous system is employed, and a .beta..sub.3 -adrenergic receptor stimulant and a fat absorption inhibitor have been developed. However, these so-called "anti-obesity drugs" are not thought to be effective for decreasing visceral fat, and hence there is no effective pharmacotherapy for decreasing visceral fat.
The relationship between insulin sensitivity and body fat distribution has been also studied, and it suggests that insulin sensitivity is caused by the visceral fat accumulation. Accordingly, it is commonly understood that visceral fat accumulation causes the above-mentioned diseases accompanied by insulin sensitivity or independently thereof.
Thiazolidine derivatives which improve insulin sensitivity are already known to effectively serve as blood glucose depressors or blood lipid depressors [Japanese Patent Provisional Publications No. 61(1986)-267580 and No. 60(1985)-51189, and WO 96/12719]. On the other hand, however, some of these derivatives are reported to promote appetite and increase the body weight. Further, it is also reported that they increase brown adipocyte in rodents and promote the differentiation of preadipocyte into adipocyte in vitro. In view of those reports, the thiazolidine derivatives are generally believed to rather increase the body fat.
Accordingly, although the relationship between insulin sensitivity and visceral fat accumulation is noticed, no insulin sensitizer serving for decreasing visceral fat has been developed.